Conventionally, for example, a hydraulic excavator dumps soil, sand, or the like in a bucket by contracting a hydraulic cylinder for the bucket, thereby turning an opening side of the bucket downward. In addition, when the hydraulic cylinder for the bucket is contracted, a piston is struck against the bottom of a cylinder tube at the stroke end of the piston to thereby cause soil, sand, or the like sticking to an inside of the bucket to fall by an impact force produced as a result of striking.
However, the impact force resulting from striking produces vibrations, which propagate to a periphery of the hydraulic cylinder and cause loud noise. More than one such impact may occur in a short time due to an elasticity of a bucket link, which may result in emitting much noise.
In order to eliminate noise, a hydraulic cylinder having a cushioning device is used. In such a hydraulic cylinder with the cushioning device, a piston slowly comes into contact with a cylinder tube at the stroke end of the hydraulic cylinder for the bucket under contraction. As a result, a sufficient impact force is not applied to the bucket and, accordingly, soil, sand, or the like sticking to the inside of the bucket do not fall.
To overcome the problems described above, a hydraulic cylinder (refer to Patent Document 1) has been proposed that produces impact forces at the stroke end of a piston and, moreover, reduces noise. In addition, to reduce noise at the stroke end of a piston, a load-bearing platform storage device (refer to Patent Document 2) and so on have been proposed.
FIG. 14 is a cross-sectional view of a configuration of the hydraulic cylinder described in Patent Document 1 as a first conventional example related to the present invention. The hydraulic cylinder 50 shown in FIG. 14 is the hydraulic cylinder 50 for the bucket. The hydraulic cylinder 50 includes a cylinder tube 51, a piston 52, and a cylinder rod 53. The bucket (not shown) is pivotally supported on a leading end of the cylinder rod 53. A trailing end of the cylinder tube 51 is pivotally supported on an arm (not shown).
The cylinder rod 53 is extended by supplying pressure oil to an oil chamber 54 on a bottom side of the cylinder tube 51. In addition, the cylinder rod 53 is contracted by supplying pressure oil to a oil chamber 55 on a head side. Extension and contraction of the cylinder rod 53 enables the bucket (not shown) to be rotated.
A configuration of the cylinder tube 51 is such that a cylinder bottom 57 and a cylinder head 58 are attached to a cylindrical body 56. The cylinder rod 53 projects beyond a hole 59 defined in the cylinder head 58. In addition, formed in the cylinder bottom 57 and the cylinder head 58 are passages 57a and 58a respectively.
The piston 52 of the hydraulic cylinder 50 is provided with a vibration attenuation member 60 which strikes against the cylinder bottom 57 at the stroke end and also attenuates vibration produced by striking. A configuration of the vibration attenuation member 60 is such that a block body 61 of a damping metal substance is attached to the piston 52 on a side of the cylinder bottom 57. At the stroke end of a contraction, the block body 61 comes into contact with the cylinder bottom 57. An example of the damping metal composing the block body 61 is Mn-0.22Cw-0.05Ni-0.02Fe.
In this configuration, when the hydraulic cylinder 50 for the bucket is contracted and the piston 52 reaches the stroke end, the block body 61 strikes against the cylinder bottom 57. The striking of the block body 61 against the cylinder bottom 57 is transmitted to the cylinder rod 53 as an impact force, which is consequently applied to the bucket. The impact force from the cylinder rod 53 is adequate to cause soil, sand, or the like sticking to the inside of the bucket to fall.
In addition, vibration produced by the striking of the block body 61 against the cylinder bottom 57, especially high frequency components of the vibration, can be absorbed and attenuated by the damping metal composing the block body 61. Specifically, a use of the damping metal prevents vibration generated by an impact from propagating to the piston 52, cylinder rod 53, and cylinder tube 51, that is, the periphery of the hydraulic cylinder, thus reducing the emission of noise.
FIG. 15 is a cross-sectional view of the load-bearing platform storing device described in Patent Document 2, which is a second conventional example related to the present invention. Specifically, FIG. 15 is a cross-sectional view of a cylinder 70 for upright or horizontal position which is mounted to aback of a load-bearing platform (not shown). By extending or contracting the cylinder 70 for upright or horizontal position, the load-bearing platform can be brought into an upright stored position or a horizontal projecting position. In the horizontal projecting position, a worker can carry goods or the like into or from a luggage compartment of a freight car via the load-bearing platform. In the upright stored position, the luggage compartment is closed.
In a typical load-bearing platform storage device, when its load-bearing platform is rotated upward from a horizontal projecting position to an upright stored position, a rotation moment at an initial stage of a rotation is large and, therefore, the load bearing platform is slowly rotated upward. However, as the rotation moment decreases with further upward rotation of the load-bearing platform, the load-bearing platform gradually increases its rotating speed and stands upright. For this reason, in the upright stored position where the rotation moment does not act, the speed of the rotation is highest. Consequently, in the upright stored position, the load-bearing platform strikes against a platform storage chamber or the like, and stops while emitting loud noise, which is a problem.
In order to solve the problems discussed above, the load-bearing storage device described in Patent Document 2 has been proposed. As shown in FIG. 15, disposed in a cylinder main body 71 of the cylinder 70 for the upright or horizontal position is a piston 73 which is freely slidable and fixed to a basal end of a rod 72. When operational hydraulic oil is supplied to an oil supply/exhaust port 74 formed in a bottom of the cylinder main body 71, the piston 73 can slide toward the head by the pressure of the operational hydraulic oil so as to extend the rod 72. A plurality of disk springs 75 are disposed inside the rod 72 on a head side of the cylinder main body 71.
When the cylinder 70 for upright or horizontal position is extended, a reaction force of the disk springs 75 does not act as extension begins. However, when the piston 73 slides toward the head and comes into contact with the disk springs 75, the reaction force of the disk spring 75 acts on the piston 73. This decelerates an extending operation of the cylinder 70 for the upright and horizontal position, so that a load-bearing platform (not shown) slowly becomes upright. When the disk springs 75 are compressed to a predetermined degree L, the cylinder 70 for upright or horizontal position reaches its maximally extended state so that the load-bearing platform is stored upright. Accordingly, in the upright stored position, the load-bearing platform slowly comes into contact with the storage chamber and stops without emitting loud impact noise.    [Patent Document 1] Japanese Patent Application Laid-Open Publication No. 2004-332778    [Patent Document 2] Japanese Patent Application Laid-Open Publication No. 11-189090